The present invention relates to an optical module, an optical imaging device comprising such an optical module as well as to a method for holding an optical element. The invention can be employed in connection with microlithography used in the production of microelectronic circuits. It also relates to an optical imaging method, which can be carried out inter alia with the optical imaging device according to the invention.
Particularly in the field of microlithography, apart from the use of components made with the highest possible precision, it is necessary inter alia to maintain the position and geometry of the components of the imaging device, thus for example the optical elements such as lenses, mirrors or grids, unchanged as far as possible during operation, in order to achieve correspondingly high reproduction quality. The high precision demands, which lie in the microscopic range in the order of magnitude of a few nanometres or below, are in this case not least a consequence of the constant need to increase the resolution of the optical systems used in the production of microelectronic circuits, in order to promote the miniaturization of the microelectronic circuits to be produced.
In order to obtain higher resolution, either the wavelength of the light used can be reduced, as is the case in systems, which work within the extreme UV range (EUV) with working wavelengths in the range of 13 nm, or the numeric aperture of the projection system can be increased. A significant increase of the numeric aperture above the value One is possible with so-called immersion systems, in which an immersion medium, whose refractive index is greater than One is provided between the last optical element of the projection system and the substrate to be exposed. A further increase of the numeric aperture is possible with optical elements having a particularly high refractive index.
Both with the reduction of the working wavelength and with the increase of the numeric aperture, not only the demands for positioning accuracy and dimensional stability of the optical elements used throughout their entire operation, increase. Also of course the demands regarding minimization of the imaging errors of the total optical arrangement increase.
Of special importance here is the intrinsic weight of the optical elements used. The lower this proves to be, on the one hand the higher the attainable resonant frequency of the arrangement consisting of the optical element and its retaining structure. Thus oscillations of the arrangement caused by mechanical disturbances and imaging errors resulting therefrom can be reduced. On the other hand, with the intrinsic weight, naturally the deformation of the optical elements caused by the intrinsic weight and the aberrations resulting therefrom, also reduce.
In the field of microlithography lenses are frequently supported by means of a plurality of spring elements, which are evenly arranged on the periphery of the respective lens and contact one of the optical surfaces of the lens in the area of the so-called overrun of the lens, that is to say, in the boundary region of the lens outside the free optical diameter of the lens. The lens in this case either lies from above on top of the spring elements or is attached from below to the spring elements. At the same time it is usually adhesively bonded with the spring elements. The spring elements, in the radial direction of the lens, are usually constructed to be as compliant as possible in order to prevent stresses—for example caused by different thermal expansion of the lens and the supporting structure—being introduced by means of the supporting structure into the lens, which might lead to imaging errors.
Although due to such a configuration as even support as possible of the weight of the lens is achieved, it has the disadvantage that for the contact zone between the spring elements and the lens in the radial direction of the lens outside its free optical diameter, a comparatively large boundary region and thus a comparatively large overrun are necessary. In other words as a result the diameter of the lens substantially increases beyond the actual dimension necessary to fulfil the optical function. Herewith, naturally, the intrinsic weight of the lens also increases.
From U.S. Pat. No. 4,733,945 (Bacich)—the disclosure of which is herewith incorporated by reference—it is also known, inter alia, to hold a lens in the radial direction by means of compliant spring elements, which are adhesively bonded with the cylindrical border of the lens. The spring elements in this case run in the circumferential direction of the lens, so that—due to the length of the spring elements in the circumferential direction—only a very limited number of spring elements can be arranged on the periphery of the lens. This has the disadvantage that, on account of the comparatively small number of spring elements, a comparatively large adhesive surface is needed for each spring element, in order to be able to support the intrinsic weight of the lens and the dynamic loads arising thereon.
Such comparatively large adhesive surfaces in turn have the disadvantage that, on the one hand, due to possible shrinkage of the adhesive, substantial stresses are introduced into the lens in a concentrated way. On the other hand the large adhesive surfaces also require a comparatively high dimension of the cylindrical border, transverse to the main extension plane of the lens, that is to say, in the direction of the optical axis of the lens. The lens must therefore be made correspondingly thick, as a result of which its overrun and thus its intrinsic weight in turn substantially increase beyond the optically required dimension.